This invention relates generally to pressure operated relief valves having opening or relief characteristics of the proportional and instant opening, i.e. "pop" variety. Relief valves having a proportional opening characteristic provide venting or relief capacity which is proportional to the difference between a predetermined or set pressure and the particular pressure acting on the valve. In contrast the instant opening or "pop" relief valves provide discharge characteristics which are a constant and maximum after the sense pressure exceeds the predetermined or set value. Two embodiments are disclosed hereinunder, a single stage proportional pilot having an adjustable opening speed, and a two stage instant opening pilot providing a non-flow characteristic during relief operation. Also, disclosed and used in conjunction with each of the above pilots is a novel method of testing set point calibration of the pressure sensitive elements.
Pilot valves providing the proportional characteristic have been used in the past. A typical prior art valve is disclosed in U.S. Pat. No. 3,477,456. The structures utilized in the disclosed valves employ more than one diaphragm and typically require that pressure of the tank to be vented be raised to the valve set pressure for testing. Other known prior art valves employ mechanical pressure balancing structure which involves high pressure moving seals of substantial size and number. These constructions, therefore, encounter increased histeresis or "stiction" in operation.
Prior art relief and/or valve combinations are disclosed in U.S. Pats. Nos. 3,595,263, 3,664,362, 3,512,560, and 3,568,706. However, although the operation provided by these combinations of main and pilot valves is acceptable, adequate control of the dynamic relationships between the pilot valve, main valve, and pressure vessel or conduit is difficult to achieve. In addition, the prior art valves, in many cases, when tested on site, provide a method of testing only a pilot valve set point. Therefore, reset or blowdown pressure setting of the pilot valve and the main valve operation remain untested. The pilot valve disclosed herein provides for on site testing of pilot set point, blowdown, or reset pressure, and main valve, or alternately the pilot valve alone, through the use of small quantities of an auxiliary test gas.
Therefore, it is an object of the invention to provide an improved two-stage pilot valve for operation of a larger capacity pressure relief valve wherein test operation of the first stage pilot is possible without operation of the main valve.
It is an object of the invention to provide a pressure relief valve of substantial capacity having improved control of the product flow and pressure dynamics associated with the pilot valve, main relief valve, and pressure vessels or conduits requiring pressure relief.
It is a further object of the invention to provide an improved two-stage pressure sensitive pilot valve, wherein flow of the product, exerting the sensed pressure, through each stage of the valve occurs only during relief or reset operation of the pilot.
It is still a further object of the invention to provide a two-stage pressure operated pilot valve for operating main relief valves of substantially larger capacity where greatly reduced flow through the pilot can be highly filtered providing increased reliability through a reduction in contamination.
It is an object of the invention to provide a single stage pilot valve for operating a larger capacity pressure relief valve, wherein product discharge of the combination is proportional to sensed over pressure.
It is an additional object of the invention to provide a relief valve having a proportional discharge characteristic wherein seal frictional forces acting on the pressure sensitive elements are minimized resulting in improved repeatability and higher sensitivity.
It is a still further object of the invention to provide a pilot valve having a proportional relief characteristic incorporating a poppet design wherein selective pressure balancing results in reduced poppet transient motion.
A further object of the invention is to provide a proportional acting pressure operated relief valve employing a pressure sensitive selector wherein pilot calibration and main valve operation can be achieved without raising product pressure to the relief setting of the combination and which consumes a minimum amount of test gas.